Conventionally, when gas carburizing is carried out on a steel treatment object, the carburizing temperature has been set below the eutectic point at which the liquid phase transforms into γ iron and cementite (for example, the temperature at point C in the iron-carbon equilibrium diagram shown in FIG. 1, which is 1147° C.). However, when the carburizing temperature is limited to below the eutectic point, the diffusion rate of carbon atoms in the austenite slows, and since a large amount of time is required to increase the carburized depth from the surface of the treatment object, the carburizing time cannot be reduced.
In order to achieve a reduction in the carburizing time, increasing the carburizing temperature to or above the eutectic point has been considered to increase the diffusion rate of carbon atoms in the austenite.
However, even when the carburizing temperature is increased to or above the eutectic point, a large amount of time is required for the surface carbon concentration of the treatment object to reach the target value, and hence it is difficult to further reduce the carburizing time.
An object of the present invention is to provide a method of gas carburizing which is capable of solving the conventional problem described above.